1. Technical Field of the Invention
The present invention pertains in general to the synchronization of a radio receiver to a carrier signal frequency and, more particularly, to a method and apparatus for controlling a frequency correction feedback loop in a radio telephone.
2. Description of Related Art
While in idle mode, a radio telephone periodically monitors a carrier signal associated with a communication channel in order to maintain frequency synchronization with the carrier signal. The carrier signal frequency, however, often varies by an offset frequency which is introduced from a variety of sources. Some of the sources are external to the radio telephone and include frequency offsets due to Doppler shifts and tolerance variations in equipment transmitting the carrier signal. On the other hand, other frequency offsets are introduced by the radio telephone itself. These sources include frequency offsets from the reference oscillator and a frequency estimator used in a frequency correction feedback loop.
Referring now to FIG. 1 there is illustrated a functional block diagram of a frequency correction feedback loop currently used in radio telephone equipment. An input carrier signal of frequency f.sub.c including an external offset frequency error .+-.f.sub.e1 enters a mixer 100 and is mixed with an output signal of the frequency correction feedback loop. The output signal of the frequency correction feedback loop includes an estimation of the input signal frequency f.sub.c .+-.f.sub.e1 and an internal offset frequency error .+-.f.sub.e1 .+-.f.sub.e2 which is introduced due to errors in the frequency control feedback loop. A primary source of the internal offset frequency error .+-.f.sub.e1 .+-.f.sub.e2 is a frequency estimator 110.
The frequency estimator 110 estimates the offset signal frequency .+-.f.sub.e1 .+-.f.sub.e2 and produces an offset frequency f.sub.o which is an estimation of the difference between the actual carrier signal frequency and an internally generated synchronization frequency. The offset frequency f.sub.o is applied to a temperature compensated reference oscillator (TCXO) 120 and controls the reference oscillator output frequency. The output signal of the reference oscillator 120 is applied to a frequency synthesizer 130 which generates the output signal f.sub.c .+-.f.sub.e2 of the frequency correction feedback loop.
Various approaches have used in an attempt to minimize the offset frequency error .+-.f.sub.e1 .+-.f.sub.e2 A first approach has been to use a more accurate frequency estimator 110 which provides a more accurate estimation. This approach, however, increases the complexity and cost of the radio telephone. A second approach has been to make a series of estimates over many bursts and then average the estimates to reduce variances in the individual estimates. Performing estimates over many bursts, however, also increases the complexity of the radio telephone and, moreover, increases power consumption in the radio telephone. A third approach involves decreasing the frequency step size of the reference oscillator 120. By reducing the frequency step size, the frequency of the reference oscillator 120 can be increased or decreased by numerous small steps in frequency. This results in a trial and error approach to arrive at the step size. This approach, however, drastically increases the complexity of the radio equipment as a result of the complex process involved in analyzing all the possible frequency combinations.
For the foregoing reasons, it would be advantageous to devise a method and apparatus which decreases the error in an estimated offset frequency generated by a frequency estimator in a frequency correction feedback loop of a radio telephone. Furthermore, it would be advantageous if such a method and apparatus did not substantially increase the complexity or power consumption of the radio telephone.